The major thrust of this work has been to examine the genetic and molecular nature of the enterotoxin gene(s) of Escherchia coli and Vibrio cholerae. The structural gene(s) for the E. coli heat-labile enterotoxin (LT) has been isolated by recombinant DNA methods. At present the nucleotide sequence for the B subunit (10,300 daltons) has been determined. The nucleotide sequence encodes for 103 amino acids, of which some 80 are identical to the published amino acid sequence of V. cholerae. The differences that do occur may be explained in most cases by a single base pair change. Currently, we are completing the nucleotide sequence of the A subunit. In addition, plasmid derivatives carrying only the B subunit of LT and only the A subunit of LT have been prepared. Strains carrying each of these derivatives are being used to immunize animals so that the precise role of anti-toxic immunity may be determined.